1. Field of the Invention
The present invention relates to a developer carrier used in a developing device for developing and visualizing a latent image formed on an image-bearing member such as an electrophotographic photosensitive member or an electrostatic recording derivative. Further, the present invention relates to a developing device and a process cartridge each of which uses the developer carrier.
2. Description of the Related Art
Up to now, various electrophotographic methods have been known. Generally with the methods, an electrical latent image is formed on an electrostatic latent image holding member (photosensitive drum) with the use of various means by using a photoconductive material; then, the electrostatic latent image is subjected to developing with a developer (toner) to be visualized; a toner image is transferred onto a transferring material such as paper as the occasion demands; and thereafter, the toner image is fixed onto the transferring material with heat, pressure etc., thereby obtaining a copied material.
Developing systems in the electrophotographic methods are mainly divided into one-component developing systems and two-component developing systems. In recent years, a copying device part needs to be reduced in size with the purpose of attaining reduction in weight and in size of an electrophotographic device. Thus, a developing device that uses the one-component developing system is used in many cases.
The one-component developing system does not require carrier particles such as glass beads or iron powder differently from the two-component developing system, and thus, reduction in size and in weight of the developing device itself can be attained. On the other hand, in the two-component developing system, a toner density in a developer needs to be maintained at a constant level, and thus, a device for detecting a toner density and supplying a necessary amount of toner is required. Therefore, a large and heavy developing device is provided here. The one-component developing system does not require such a device, and thus, is preferable in the point that a developing device can be reduced in size and in weight.
As the developing device using the one-component developing system, the following one is known. With the device, first, an electrostatic latent image is formed on a surface of a photosensitive drum serving as an electrostatic latent image holding member; a positive or negative charge is imparted to a toner through friction between a developer carrier (developing sleeve) and the toner and/or a developer layer thickness regulating member for regulating a toner coating amount on the developing sleeve and the toner; then, the toner imparted with the charge is thinly applied on the developing sleeve, and is fed to a developing region where the photosensitive drum and the developing sleeve face to each other; the toner is flied and adhered to the electrostatic latent image on the surface of the photosensitive drum in the developing region, whereby the electrostatic latent image is visualized as a toner image.
However, in the case of using the above-mentioned one-component developing system, charging property of the toner is difficult to be adjusted. Although various devices on the toner are implemented, the problems on nonuniformity of toner charging and endurance stability of charging have not been completely solved.
In particular, there tends to occur, specially under low humidity, a so-called charge-up phenomenon: in which a charging amount of the toner coated onto the developing sleeve is excessively increased due to the contact with the developing sleeve while the developing sleeve rotates repeatedly; then, the toner and the surface of the developing sleeve attract each other due to a reflection force therebetween so that the toner is fixed on the developing sleeve surface; and the toner does not move to a latent image on the photosensitive drum from the developing sleeve. When the above-mentioned charge-up phenomenon occurs, the toner as an upper layer is difficult to be charged, and a developing amount of the toner is reduced. Thus, the problems of thinning of a line image, reduction in image density of a solid image, and the like arise. Further, there occurs a so-called blotch phenomenon in which: the toner, which is not properly charged due to charge-up, is failingly regulated and flows onto the sleeve; and the toner is formed into spotted or wave-shape unevenness.
Further, the respective formation states of a toner layer are changed in an image portion (toner consumption portion) and a non-image portion, so that the charging states differs therebetween. Therefore, there tends to occur a so-called sleeve ghost phenomenon in which, for example, when the position where a solid image with a high image density has been developed once on the developing sleeve corresponds to the development position in the next rotation time of the developing sleeve and a half-tone image is developed at the developing position, a mark of the solid image appears on the image.
Moreover, reduction in particle diameter and reduction toward finer particle of the toner are promoted for the purpose of realizing digitization of electrophotographic devices and higher image quality. For example, in order to improve resolution and character sharpness and faithfully reproduce the latent image, there is generally used a toner with a weight average particle diameter of about 5 to 12 μm. Further, from the viewpoint of ecology, with the goal of attaining the further reduction in weight, size, etc. of the device, the following improvement of transfer efficiency of the toner is promoted in order to decrease a waste toner. For example, a transfer efficiency enhancer with an average particle diameter of 0.1 to 3 μm and hydrophobic silica impalpable powder with a BET specific surface area of 50 to 300 m2/g are made to be contained in a toner, whereby the volume resistance of the toner is reduced, and a thin film layer of the transfer efficiency enhancer is formed on the photosensitive drum. As a result, the transfer efficiency is enhanced. Further, the toner itself is processed to have a spherical shape with a mechanical impact force, and thus, the transfer efficiency is improved.
Furthermore, there is a tendency that a toner fixation temperature is lowered with the purpose of attaining the reduction of a first copy time and the saving electricity. Under such circumstances, in particular, the toner under low temperature and low humidity is easy to electrostatically adhere onto the developing sleeve because the charge amount per unit mass of the toner increases; on the other hand, the toner under high temperature and high humidity is easy to be changed in quality due to a physical force from the outside or because of the fact that the toner is made of a material apt to be fluidized. Therefore, sleeve contamination and sleeve fusion are easy to develop.
As a method of solving the above-mentioned phenomena, there is proposed, in JP 02-105181 A, JP 03-036570 A, and the like, a method that uses a developing sleeve that is formed by providing a coating layer, which is made by dispersing conductive impalpable powder such as crystalline graphite and carbon in resin, on a metal substrate. It is recognized that the above-mentioned phenomena are significantly reduced by using the method.
However, in the case of the addition of a large amount of the powder, the method is effective in avoiding the occurrence of charge-up and sleeve ghost. However, moderate charging imparting ability to the toner is insufficient, and a sufficient image density is difficult to be obtained particularly in a high-temperature and high-humidity environment. Further, in the case of the addition of the large amount of the powder, the coating layer becomes brittle and easy to be scraped off, and also, the shape of the layer surface becomes nonuniform. Thus, in the case where the endurable use proceeds, surface roughness and surface composition of the coating layer are changed, and feeding failure of the toner and nonuniformity of charge impartation to the toner occur easily.
In the case of using the coating layer in which the crystalline graphite is dispersed, the surface of the coating layer has lubricity that arises from the scaly structure of the crystalline graphite. Thus, the coating layer sufficiently exhibits an effect on the prevention of the occurrence of charge-up and sleeve ghost, but the scaly shape makes the surface shape of the coating layer nonuniform. Further, since the hardness of the crystalline graphite is low, wear and desorption of the crystalline graphite itself are easy to occur on the coating layer surface. In the case where the endurable use proceeds, surface roughness and surface composition of the coating layer are changed, which may easily lead to feeding failure of the toner and nonuniformity of charge impartation to the toner.
On the other hand, in the case where the addition amount of the conductive impalpable powder in the coating layer formed on the metal substrate of the developing sleeve is small, the effect of the conductive impalpable powder such as crystalline graphite and carbon is limited. Thus, such a problem is left in that the measures against charge-up and sleeve ghost are insufficient.
Further, in JP 03-200986 A, there is proposed a developing sleeve in which a conductive coating layer, in which conductive impalpable powder such as crystalline graphite and carbon, and further spherical particles are dispersed in resin, is provided on a metal substrate. With the developing sleeve, wear-resistance of the coating layer is enhanced to some extent, the shape of the coating layer surface is made uniform, and change in surface roughness due to endurable use is relatively small. Therefore, toner coating on the sleeve is stabilized, and toner charging can be made uniform up to a point. As a result, there arises no problem on sleeve ghost, image density, image density unevenness, and the like, and there is a tendency of image quality to be stabilized. However, even the developing sleeve is insufficient for stabilization of moderate charging imparting ability to a toner, and quick and uniform charging controllability to a toner. Further, in terms of wear-resistance as well, the change in roughness and nonuniformity in roughness of the coating layer surface, which arise from wear or desorption of the spherical particles and crystalline graphite contained in the coating layer in the developing sleeve, and the following toner contamination, toner fusion, and the like on the coating layer occur due to the further endurable use over a long term. In this case, toner charging becomes unstable, which becomes the cause of image defect.
Further, proposed in JP 08-240981 A is a developing sleeve in which: conductive spherical particles with low specific gravity are uniformly dispersed in a conductive coating layer, thereby enhancing wear-resistance of the coating layer and making the shape of the coating layer surface uniform, which increases uniform charging imparting property to a toner; and toner contamination and toner fusion are suppressed even when the coating layer is somewhat worn. However, even the developing sleeve is incomplete in point of quick and uniform charging imparting property to a toner and moderate charging imparting ability to a toner. Moreover, as to the wear-resistance as well, the conductive particles such as the crystalline graphite are apt to wear and fall off from the portion where the conductive spherical particles do not exist on the coating layer surface in the further endurable use over a long term. The wear of the coating layer is promoted from the portion where the particles wear and fall off, whereby toner contamination and toner fusion are caused. As a result, toner charging becomes unstable, which becomes the cause of image defect.